I think you'd have to check out the target branch (stashing any changes if you had them), do `git merge source_branch`, and then check out the original branch you were on, and pop stash if you saved one.

Alternatively if the merge is a simple fast-forward you can just do `git push . source:destination`, which is what `git forward-merge` does in that case too.

Works only for fast-forward merges:

    git fetch origin master:master